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Date: Fri, 16 Mar 2018 15:18:23 +1000
From: Nicholas Piggin <npiggin@gmail.com>
To: Michal Suchanek <msuchanek@suse.de>
Cc: linuxppc-dev@lists.ozlabs.org, Kate Stewart
 <kstewart@linuxfoundation.org>, Madhavan Srinivasan
 <maddy@linux.vnet.ibm.com>, Paul Mackerras <paulus@samba.org>, Michael
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 Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>, "Naveen N. Rao"
 <naveen.n.rao@linux.vnet.ibm.com>, Daniel Axtens <dja@axtens.net>, Al Viro
 <viro@zeniv.linux.org.uk>, David Gibson <david@gibson.dropbear.id.au>, Greg
 Kroah-Hartman <gregkh@linuxfoundation.org>, linux-kernel@vger.kernel.org,
 Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, =?UTF-8?B?Q8OpZHJpYw==?=
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 <oohall@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, "Tobin C.
 Harding" <me@tobin.cc>
Subject: Re: [PATCH RFC rebase 3/9] powerpc/64: Use barrier_nospec in
 syscall entry
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On Thu, 15 Mar 2018 20:15:52 +0100
Michal Suchanek <msuchanek@suse.de> wrote:

> On powerpc syscall entry is done in assembly so patch in an explicit
> barrier_nospec.

Same comment as Linus for this -- the barriers are before the branch here,
so is it possible the branch instruction can be speculative while the index
is used to load the syscall table?

Thanks,
Nick

> 
> Signed-off-by: Michal Suchanek <msuchanek@suse.de>
> ---
>  arch/powerpc/kernel/entry_64.S | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> index 2cb5109a7ea3..7bfc4cf48af2 100644
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -36,6 +36,7 @@
>  #include <asm/context_tracking.h>
>  #include <asm/tm.h>
>  #include <asm/ppc-opcode.h>
> +#include <asm/barrier.h>
>  #include <asm/export.h>
>  #ifdef CONFIG_PPC_BOOK3S
>  #include <asm/exception-64s.h>
> @@ -159,6 +160,7 @@ system_call:			/* label this so stack traces look sane */
>  	andi.	r11,r10,_TIF_SYSCALL_DOTRACE
>  	bne	.Lsyscall_dotrace		/* does not return */
>  	cmpldi	0,r0,NR_syscalls
> +	barrier_nospec
>  	bge-	.Lsyscall_enosys
>  
>  .Lsyscall:
> @@ -319,6 +321,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  	ld	r10,TI_FLAGS(r10)
>  
>  	cmpldi	r0,NR_syscalls
> +	barrier_nospec
>  	blt+	.Lsyscall
>  
>  	/* Return code is already in r3 thanks to do_syscall_trace_enter() */